Optically mapping the electronic structure of coupled quantum dots

Abstract

In a network of quantum dots 1 embedded in a semiconductor structure, no two are the same, and so their individual and collective properties must be measured after fabrication. Here, we demonstrate a 'level anti-crossing spectroscopy' (LACS) technique in which the ladder of orbital energy levels of one quantum dot is used to probe that of a nearby quantum dot. This optics-based technique can be applied in situ to a cluster of tunnel-coupled dots, in configurations similar to that predicted for new photonic or quantum information technologies 2-5 . Although the lowest energy levels of a quantum dot are arranged approximately in a shell structure 6-10 , asymmetries or intrinsic physics-such as spin-orbit coupling for holesmay alter level splittings significantly 11 . We use LACS on a diatomic molecule composed of vertically stacked InAs/GaAs quantum dots and obtain the excited-state level diagram of a hole with and without extra carriers. The observation of excited molecular orbitals, including σ and π bonding states, provides fresh opportunities in solid-state molecular physics. Combined with atomic-resolution microscopy and electronicstructure theory for typical dots, the LACS technique could also enable 'reverse engineering' of the level structure and the corresponding optical response 12 .To begin, we recall the previously established spectroscopic features of quantum dot molecules (QDMs). We use the recombination of an electron-hole pair (that is, an exciton X 0 ) as an indicator for the state of the hole. In these structures, the electron is localized in the bottom dot (B dot) over the entire electric field range [13][14][15][16] . Recombination with the hole within the same dot leads to an intense spectral line 1 0 1 0 in the photoluminescence spectrum ( Fig. 1). As the electric field is scanned, this intradot transition goes through the first of a series of anticrossings at a field that we take as F = 0. Here, there is a resonance with the lowest interdot transition, 1 0 0 1 , in which the hole is in the top-dot (T-dot) ground state. The interdot transition energy exhibits a strong field dependence (Stark shift), caused by a change in the relative level alignment between the two dots with electric field. The strength of the shift is determined by the dot separation. Its slope ( E/ F = 0.955 meV kV −1 cm) provides a built-in calibration for the conversion between electric field ( F) and energy ( E). This first resonance arises from the coherent tunnelling of the hole between the 's shells' of the two dots (B0 and T0), and the corresponding formation of a bonding and an antibonding molecular state. Such resonances between the ground states of two quantum dots have been studied intensely in recent publications [13][14][15][16][17][18][19][20][21][22][23] . Energy (meV) B-T-dot T0 CB VB 1 2 3 4 Intradot B0 Intradot Interdot 1 0 0 1 ΔE T0 T1 T2 T3 T4 1 1,252 1,253 ΔF --( ) 1 0 1 0 --( ) 1 0 1 1 --( ) 0 2 0 4 0 -3.0 -2.5 -2.0 -3.5Relative electric field (kV cm -1 )Applied bias (V) Figure 1 Principle of LACS. The ...

Scalable qubit architecture based on holes in quantum dot molecules

“…Spectroscopy of the excited states of holes in QDMs suggests that this limit would permit tuning over approximately 10 meV. 40 Tuning over 10 meV is an order of magnitude improvement over the typical Stark shift tuning (of order 1 meV) achieved for single InAs QDs 14 and comparable to the giant Stark shift that can be achieved for single QDs confined between AlGaAs barriers. 41 In our QDM design the AlGaAs layer that blocks injection of carriers from the n-type GaAs can be moved arbitrarily far away from the QDs.…”

Optophononics with coupled quantum dots

“…The height of dots in both layers was nominally set to 2.5 nm. Dots in the second layer were found to exhibit higher band gaps than dots in the first layer, which is attributed to a larger lateral growth of dots in the second layer causing a different evolution of the dots during the indium flush [46][47][48] .…”

Origin of two-hole triplet splitting in circular quantum dots

“…Soon after it was realised that single-hole p-shell energy levels in InAs/GaAs QDs also present few-meV splittings. [9,10] These splittings could be easily understood if the lateral confinement was anisotropic, e.g. due to an elongation of the QD base or to piezoelectric fields.…”